The external propellant tank of the space shuttle, designated 10 in FIG. 1a, has been successfully made from aluminum. The external propellant tank 10 includes a hydrogen propellant tank 11, which is made from two end pieces 14 and 15, each of which is in the general form of a hemisphere, separated by two or more cylindrical tank sections, one of which is designated 16, and all of which are welded together. The complete tank 10 includes an intertank 12 and oxygen tank 13. An automated arrangement for welding sections of the tank is described in U.S. Pat. No. 5,483,039, issued Jan. 9, 1996 in the name of Gallagher. The standard aluminum alloy external tank is, however, a significant contributor to the weight of the space shuttle at launch. In order to improve the payload of the space shuttle, attention has been directed toward fabricating the external tank from lighter-weight materials than standard aluminum alloys. Among the materials which are u under investigation is aluminum-lithium 2195 alloy, which is light and strong. Unfortunately, aluminum-lithium alloys are more difficult to fabricate than ordinary aluminum alloys, in that welding tends to change the structure of the material in the region of the weld, as mentioned in U.S. patent application Ser. No. 08/584,901, filed Jan. 11, 1996 in the name of McGee et al.
Improved welding and repair methods are desired.